San Diego County faces possible Valentine's Day drenching from 'atmospheric river'

Gary Robbins/Union-Tribune

A weak Pacific storm moved through Southern California on Sunday, producing showers at the coast and snow in the mountains.

A weak Pacific storm moved through Southern California on Sunday, producing showers at the coast and snow in the mountains. (Gary Robbins/Union-Tribune)

Gary Robbins

A river of moisture from the subtropics might flow through San Diego County during the middle of the week, dropping heavy rain and snow in the mountains and light-to-moderate precipitation at the coast.

The National Weather Service says it appears that the moisture will flow up mountain slopes, condense, and produce heavy precipitation — a phenomenon known as orographic lift.

But it is unclear whether a cold front will drop in from the North Pacific to draw the moisture ashore and into the mountains.

The system is expected to arrive late Wednesday and could hit with force on Thursday — Valentine’s Day.

“The models aren’t showing heavy rain at the coast, but the mountains could get a lot of rain and snow,” said Joe Cordero, a weather service forecaster.

“If we do get a storm, it will feel warmer because the moisture is coming from the subtropics.”

Cordero added that the latest models show the atmospheric river tracking into the Sierra Nevada, “where it will go off like a bomb.”

That could be good news for Californians — if the moisture falls as snow. But a warm storm could produce heavy rain in the Sierra that would partially melt an already substantial snow pack, which could create flooding worries.

Since the rainy season began on Oct. 1, San Diego International Airport has recorded 9.11 inches of rain, which is 3.37 inches above average. Forecasters said it is possible that the airport will surpass its average annual rainfall mark of 10.34 inches by the end of February.